Zeroth Law of Robotics

In Robots and Empire (pg. 63), the "Zeroth Law" is extrapolated, and the other Three Laws modified accordingly. Unlike the Three Laws, however, the Zeroth Law is not a fundamental part of positronic robotic engineering, is not part of all positronic robots, and, in fact, requires a very sophisticated robot to even accept it.

The Zeroth Law is also mentioned in one of the last Foundation books that Asimov wrote, Foundation and Earth. The robot R. Daneel Olivaw, is found by Foundation explorers. He is the only robot in the Foundation universe (so far as I know, anyway), and because of his exceptionally long existence and high intelligence, he has altered his own programming (originally only containing the usual three laws) to accommodate the Zeroth Law.